Sending Gift Voucher delay for page to respond

I've recently had a site upgrade so am running the latest Zen Cart.

However when ever I try and send a gift voucher via admin, it now takes 3-4 minutes for the page to respond (most frustrating!)

Can anyone shed light as to why a delay now occurs? I never had any delay in 1.3.7

How many customers do you have?

6500 customers

Most likely the slow down is the dropdown of the 6500 lines needed for the out put ...

In the file:
/admin/gv_mail.php

if you comment out:

              <tr>
                <td class="main"><?php echo TEXT_CUSTOMER; ?></td>
                <td><?php echo zen_draw_pull_down_menu('customers_email_address', $customers, $_GET['customer']);?></td>
              </tr>

then the 6500 line dropdown will not display and the screen will draw much faster ...